1. Field of the Invention
The present invention relates to a method and a device for charging a battery system having a plurality of voltage sources.
2. Description of Related Art
Published German patent document DE 101 50 376 describes a device for equalizing the charge condition of accumulators connected in series. According to this approach, a capacitor is provided, as well as a plurality of switches situated between the capacitor and the accumulators. Furthermore, the device has a logic which is used for measuring the voltages applied to the accumulators and, when a predefined differential threshold value between the voltages applied to the accumulators is exceeded, for activating a driver controlling the switches. The capacitor is connectable for the purpose of a charge exchange alternatingly with the individual accumulators via the switches. In addition, the device known from published German patent document DE 101 50 376 has two accumulators connected in series and four switches. A first of the switches is situated between a first terminal of the capacitor and the terminal of the accumulators connected in series, farther away from the ground, a second of the switches is situated between the second terminal of the capacitor and ground, a third of the switches is situated between the second terminal of the capacitor and the point of connection between the two accumulators, and a fourth of the switches is situated between the first terminal of the capacitor and the point of connection between the two accumulators.
If a plurality of battery cells is connected in series in order to increase the output voltage of the battery system, the voltage across the individual cells is established, among other things, as a function of their “health condition” or of the prevailing internal resistance, in addition to other influencing factors when the battery system is charged. If battery cells which are sensitive to overvoltages are used, the charging operation of the battery system is terminated when the maximum allowable cell voltage is attained at one of the individual cells connected in series.
The more homogeneously the individual cells are able to be charged, the higher the service life of the battery system. Since, during the charging operation, there is inhomogeneity within the individual cells of the battery system, in which the maximum allowable cell voltage value is attained before other individual cells attain this value, the charging operation is switched off, although some of the cells are not yet fully charged. On discharging, the not fully charged individual cells initially attain a predefined lower voltage value and the discharging operation is terminated, although individual cells in the battery system which are capable of outputting their charge are still available. This effect of inhomogeneity may become reinforced over the operating time and as a function of the number of charging and discharging cycles to which the battery system is subjected. The greater the inhomogeneity, the smaller is the difference between available and chargeable energy. Due to the above-described effect, the battery system is no longer able to be used effectively and must therefore be replaced. In general, this effect of inhomogeneity is counteracted by a careful selection of the individual cells having identical characteristics for serial circuits in a battery system, and by using a charge transfer in which a charge is transferred from individual cells which are in a better condition to individual cells which are in a worse condition via transformer circuits. In contrast, the selection of the individual cells represents a relatively high degree of complexity. Transformers, which were previously used for this purpose, require a large installation space and are expensive. Furthermore, removal of heat during the homogenization operation requires expending a considerable effort.